Abstract
Blood flow (BF) due to muscle contractions during exercise providing information about increased oxygen supply and energy metabolism, which may be the underlying mechanism of exercise therapy for cardiovascular disease. The same mechanism also normalizes the central and peripheral hemodynamics in body. High time resolution Doppler ultrasound measurements have been reported to detect not only the beat-to-beat blood velocity profile in the nonmuscular contraction state (basal resting condition), but also alterations in blood velocity profile in exercising limb during muscle contractions. During sustained isometric (static) muscle contraction (IMC), the systole-diastolic blood velocity profile is not intermittently disturbed due to the absence of fluctuations of intramuscular pressures compared to isotonic (dynamic) muscle contractions. This offers the possibilities to evaluate the blood velocity and BF magnitude for each beat-to-beat cycle without the influence of rapid changes in intramuscular pressure. Our previous studies demonstrated that elevated intramuscular pressure during 10 s-sustained IMC may transiently restrict muscle hyperemia during steadystate repeated knee extensor exercise at 3 contractions per minute (10 s-IMC and 10 s-muscle relaxation) at 10%, 30%, 50% and 70% of maximum voluntary contraction. In contrast, the sudden increase in peak BF immediately after the end of a IMC steeply declines during the period of 10 s-muscle relaxation. However, there is a lack of information regarding the extent to which the beat-to-beat magnitude in BF during IMC and muscle relaxation, even if large differences are observed in the BF magnitude between IMC and relaxation phases. We provided that the time-dependent beat-to-beat BF magnitude may significantly (P<0.05) linearly correlate for both 10 s-IMC and 10 s-relaxation phase. Furthermore, the variation in the acceleration rate of time dependent changes in BF is small in individual subjects below 30% of maximum voluntary contraction both IMC and relaxation phase. The present commentary therefore discusses to what extent the time-dependence of beat-to-beat magnitude in BF may statistically correlate during repeated IMC and relaxations with supplementary consideration of isometric exercise induced BF changes and its evaluation.

Keywords: Exercising blood flow; Isometric muscle contraction and relaxation; Doppler ultrasound